1. Field of the Invention
The invention concerns a circuit arrangement for actuating a VFD, LED or LCD display.
2. Discussion of the Prior Art
Such displays are widely used for example in small-scale electrical devices. They serve inter alia for displaying clock time or temperature. In the case of electronic cooker time switches, such displays additionally bear symbols for the set mode of operation of the cooker or oven or for other functions such as alarm or radio reception. A numerical display (for example for the clock time) is here usually implemented by means of a 7-segment display.
The actuation of such displays is effected in different ways in each case according to the nature of the respective display (VFD, LED, LCD). In the case of a vacuum fluorescence display (VFD), a heated cathode is disposed behind the digits to be displayed. The individual, mutually corresponding segments of the various (7-segment) digits are respectively connected in parallel with each other and form the anodes. Disposed between the heated cathodes and the segments serving as anodes are grids which each extend over the region of a digit. A segment of a given digits lights up when both the anode corresponding to that segment and also the grid corresponding to that digit are at positive voltage. Therefore each individual segment of each individual digit can be actuated separately by way of multiplex actuation of the segments and the digits.
In the case of a light emitting diode display (LED) the individual segments of the digits are respectively formed by individual light emitting diodes. The individual, mutually corresponding segments of the various digits are again connected in parallel to each other and form the cathode while the various digits form the anodes. In this case also each segment can be addressed separately by means of multiplex actuation.
The difference in actuation as between a VFD display and an LED display is on the one hand that, in the case of the VFD, either positive voltage or no voltage is applied to the terminals while positive voltage (for digits) or negative voltage (in the case of segments) or no voltage is applied to the terminals of the LED display. On the other hand, a VFD is operated at a relatively high voltage of 16 to 30 V, but with a low current of 0.1 to 2 mA, while an LED display requires only a low voltage of about 2 V but a higher current of 2 to 20 mA.
In contrast actuation of a liquid crystal display (LCD) is effected in a fundamentally different manner. This involves passive elements which are disposed on a substrate which provides for backscattering of the ambient light. When electrical voltage is applied to such an element, the crystals contained therein are oriented whereby the ambient light is absorbed at that location, whereby the element appears dark. In order to maintain that effect for a relatively long time however the element has to be recharged repeatedly (about every 100 ms). The voltage which in that situation is applied to the element must exceed a given minimum value in order to cause orientation of the crystals.
Now an LCD is constructed in such a way that up to 3 segments are connected in parallel with each other and thus form the one electrode. The other electrode is formed by three back electrodes (COM) which are each associated with a respective one of the three segments which are connected in parallel with each other. In this case also that again permits multiplex actuation. It will be noted however that in this case it is not just two voltage values (positive or negative and zero) which are applied to the individual terminals of the display, but also intermediate voltage values (in the described case, ⅓ and ⅔ of the voltage), so that specifically controlled recharging of individual elements is possible without influencing the other elements.
While actuation of a VFD and an LED display takes place in a similar manner and can be effected with an actuating circuit if the components are of suitably generous dimensions, a basically different actuating circuit is required for the actuation of a LCD. On the other hand however it would be desirable to have a single actuating circuit which is flexibly suitable for VFD, LED and LCD displays. That would result in an enhanced degree of component standardisation, a reduction in the complication and expenditure in terms of logistics and storage and a more flexible reaction to customer wishes.